1. Field of the Invention
The present invention relates generally to rock bits for drilling oil wells and more particularly to improving the toughness of sintered tungsten carbide or polycrystalline diamond inserts used as the cutting elements in rock bits.
2. Description of the Prior Art
This invention involves an improvement over powder metallurgy composite materials of the type disclosed in U.S. Pat. No. 2,731,710. This type comprises grains of relatively hard abrasion resistant material, such as tungsten carbide, and a binder material, such as a cobalt alloy, for binding the grains together. Such composite material, which is also referred to as a cemented carbide is widely used for cutting tools as the cutting elements. These cutting elements are commonly known as inserts and are utilized in rolling cutter drill bits such as shown in U.S. Pat. No. 2,687,875, for use in drilling oil and gas wells.
While tungsten carbide with a cobalt alloy bindeer has been the standard composite material for use in inserts in the drill bit manufacturing industry for the past 30 years, this material suffers from a shortcoming. More particularly, excessive wear and breakage of inserts, reduces the useful life of the rock bits. Insert wear is often attributable to the failure of the cobalt binder to hold the tungsten carbide grains together under the relatively high compressive loads applied to the drill bit during drilling operations. Life of drill bit inserts formed of conventional cemented carbide material is substantially affected also by the lack of adequate fracture toughness and resistance to fatigue crack growth of the inserts made with a cobalt binder.
Thus, the rock bit designer is forced to make tradeoffs in selection of carbide grades, to balance breakage with wear. In rock formations where excessive breakage of inserts occurs, the designer is forced to select carbide grades with higher cobalt content, of consequently lower hardness and wear resistance with slightly increased fracture toughness. Thus, up to now, high hardness and wear resistance of inserts coupled with high fracture toughness did not appear possible.
The prior art has used means to increase conformance of the insert hole dimensions to the size and shape of inserts, to reduce tendency for insert loss and breakage caused by inserts physically being rocked during loading and then being pulled out of bits. U.S. Pat. No. 4,211,508 discloses a concept of roughening the surface finish of inserts to improve retention and consequently reduce insert loss. U.S. Pat. No. 3,581,835 discloses means for molding a polygonal shaped carbide with at least 12 sides, and prehoning the sintered article to an inward taper. This teaching uses a prehoning step in lieu of conventional grinding techniques, before tumbling and pressing into holes.
Our invention follows conventional insert manufacturing steps followed in a method disclosed that enhances the hardness, toughness, breakage resistance and strength all concurrently, as will become clear from this disclosure. The conventional insert manufacturing steps used in our industry are described here for reference. Insert manufacture typically consists of mixing blends of tungsten carbide, cobalt powders, wax and lubricants such as acetone or heptane in high energy ball mills or attritor type mills. The grade powder blends may sometimes be cast into ingots with wax, further crushed and blended in v-type blenders to ensure mixing. The process powder is then pressed into desired forms using molds/dies. The powder compacts are then sintered in a furnace cycle, the initially includes a dewax cycle followed by sintering to a liquid-phase region. Sintered parts are sometimes subjected to a further hot-isostatic pressure (HIP) treatment for further densification of the compact. Alternatively, the sintering and HIP cycles may be combined into a single sinter-HIP process. Inserts made in the described manner are then generally ground in a close tolerance operation to obtain accurate diameters and eliminate any concavity present. Thereafter, inserts are generally tumbled in an abrasive medium for a limited time of 15 to 30 minutes, to remove any scale, residue of tray coatings or surface oxidation present and to smooth sharp corners. This practice of insert manufacture has generally been followed by all insert manufacturers for downhole tools.
Extensive research efforts have been made to study the metallurgical characteristics and nature of this essentially two-phase composite material. A study carried out at Terratek laboratories is cited here for reference.
Experiments in this research effort at TerraTek Laboratories in Salt Lake City, Utah, apparently induced residual compressive stresses by ball milling inserts, to produce a marginal increase in apparent fracture toughness, as measured in the laboratory test. This was attributed to the inducement of residual compressive stresses during ball milling that acted against tensile loads used in testing.
Although these concepts have improved results somewhat, insert breakage remains as one of the biggest problems in affecting drill bit life.